/images/PowerCableUpdate.jpg/images/southwire_100x70.jpgOne Layer or Three? Unishield - Tape Shield Comparison OneLayerOrThreeUnishieldTapeShieldComparison.htmAn integrated shielding/jacket system from the 70s still holds a niche in a market dominated by helical tape shielding.
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One Layer or Three? Unishield - Tape Shield Comparison

An integrated shielding/jacket system from the 70s still holds a niche in a market dominated by helical tape shielding.

  “Helical-tape shielding has been a standard choice for medium-voltage (MV) cable for several decades,” says Doug Ramsey, Southwire director of industrial products. Helical-tape shielding commands a dominate share of the market. There is an alternative construction, known as Unishield, which was developed in the early 70s available in the market. Here’s a comparison of some of the features of the two designs.

Unishield puts several functions in a single layer

Both constructions use a semiconducting shield layer between a copper conductor and a layer of Ethylene Propylene Rubber (EPR) insulation. Above the insulation, the designs differ.

  • Helical tape designs add a semiconducting insulation shield, a helically wrapped copper tape that serves as a ground return path, and an 80-mil non-conductive jacket that delivers overall mechanical protection and also tightly secures the tape shielding during bending.
  • Unishield designs add only a single, semiconducting layer on top of the insulation. This semiconducting layer performs as both insulation shield and protective jacket. Copper drain wires embedded in the semiconducting layer provide a ground return path.

Bending radius

In theory, the flexibility of the Unishield drain wires and single outer layer might allow a somewhat tighter bending radius for a given conductor size than tape shielding. But field experience doesn’t always bear this out. When Unishield cables are bent sharply during pulling, the drain wires – which are typically covered by only about 20 mils of semiconductor – may pop out of the outer layer, or cut through the inner surface and into the insulation. That leaves the cable open to moisture intrusion, or to electrical stress concentrations in the insulation. Either condition can lead to premature cable failure.

Overall diameter

In general, Unishield cables are a few percent smaller in diameter for a given conductor size than tape-shielded constructions. If you are retrofitting cables into existing conduits or ducts, this difference may let you put in slightly larger conductors with Unishield cables. If you’re designing a new installation, choosing a larger conduit equalizes the size differential.

Consistent ground return performance

One of the motives for the development of the Unishield design in the 70s was concern that helical tape shielding might not maintain consistent electrical performance because of changes in contact resistance between the turns of copper tape as the cable aged. At that time, standard practice was to wrap helical tape with a 12.5 percent overlap between turns. More current helical tape designs – including all of Southwire’s helical tape shielding – provide a 25 percent overlap between the turns of tape.

The increased metal contact with larger tape overlap gives a greatly increased safety margin against changes in contact resistance due to aging. The higher contact area also yields greater short-circuit capacity – as much as 22 percent more than a comparable Unishield drain wire system.

“There are safety comparisons to consider also, such as the energization hazard of the semiconducting Unishield outer layer in single-point grounding systems, and the availability of low-smoke, non-halogen compounds,” Ramsey remarks. “For more details on comparisons between the two designs, contact your Southwire representative.”

(® Unishield is registered trade mark of General Cable)

Contact: Doug Ramsey